With the spread of portable electronic equipment, such as cell phones and notebook-sized personal computers, a small and lightweight secondary battery having a high energy density has been desired. Lithium-ion secondary batteries are secondary batteries suitable for such usage, and research and development thereof have been actively conducted.
Furthermore, also in the automobile field, from the view point of resource and environment, a demand for electric vehicles has been growing and thus, as a power source for electric vehicles and hybrid vehicles, there has been desired a lithium-ion secondary battery which is small and lightweight, has a large discharge capacity, and has good cycle characteristics. Particularly, output characteristics are important for a power source for automobiles, and accordingly a lithium-ion secondary battery having good output characteristics has been desired.
A 4V class high voltage can be achieved by a lithium-ion secondary battery which uses lithium-containing composite oxide, particularly uses lithium-cobalt composite oxide (LiCoO2), being relatively easily synthesized, as a positive electrode material, and therefore the commercialization of the lithium-ion secondary battery as a battery having a high energy density has been progressing. Research and development of the lithium-ion secondary battery using such lithium-cobalt composite oxide have been actively conducted to achieve excellent initial capacity characteristics and cycle characteristics, and various results have been already obtained.
However, lithium-cobalt composite oxide is produced by using an expensive cobalt compound as a raw material, and therefore an increase in cost of the active material and moreover a battery is caused, and accordingly improvement of the active material has been desired. A battery using the lithium-cobalt composite oxide costs considerably higher per capacity than a nickel-metal hydride battery, and therefore is of very limited application. Hence, not only for small secondary batteries for portable electronic equipment currently diffused, but also for large-sized secondary batteries for electric power storage, electric vehicles, and the like, there are great expectations for a decrease in cost of the active material and the resulting realization of manufacturing more inexpensive lithium-ion secondary batteries, and thus it can be said that such realization is industrially significant.
Here, as a new material for a positive electrode active material for lithium-ion secondary batteries, in recent years a 4V class positive electrode active material has attracted attention, which is more inexpensive than lithium-cobalt composite oxide, namely, lithium-nickel-cobalt-manganese composite oxide having a composition of Li(Ni1/3Co1/3Mn1/3)O2, the composition being substantially such that an atomic ratio of nickel:cobalt:manganese is 1:1:1. Lithium-nickel-cobalt-manganese composite oxide is not only inexpensive, but also has higher thermal stability than a lithium-ion secondary battery which uses lithium-cobalt composite oxide or lithium-nickel composite oxide as a positive electrode active material, and therefore the development thereof has been actively conducted.
In order for a lithium-ion secondary battery to demonstrate good battery characteristics, lithium-nickel-cobalt-manganese composite oxide serving as a positive electrode active material needs to have an appropriate particle diameter and an appropriate specific surface area, as well as a high density.
Furthermore, properties of the surface of the active material in which an electrolyte and lithium ions are interchanged are also important, and little adhesion of impurities, particularly carbon, to the surface is required. Such properties of the positive electrode active material are strongly affected by properties of nickel-cobalt-manganese composite hydroxide serving as a precursor, and therefore, the composite hydroxide is also required to have the same properties.
For the composite hydroxide serving as a precursor of a positive electrode active material, various proposals mentioned below have been made. However, there is a problem that, in any of those proposals, a material having a sufficiently high density has not been achieved, in addition, properties of the surface thereof have not been fully taken into consideration.
For example, Patent Literature 1 discloses that a nickel salt solution containing cobalt salt and manganese salt, complexing agent, and alkali metal hydroxide are continuously fed into a reaction vessel in an inert gas atmosphere or under the presence of a reductant, and the resulting crystals are continuously grown and continuously collected, whereby high-density cobalt-manganese coprecipitated nickel hydroxide is obtained, the nickel hydroxide being spherical and having a tap density of not less than 1.5 g/cm3, an average particle diameter of 5 to 20 μm, and a specific surface area of 8 to 30 m2/g. The obtained coprecipitated nickel hydroxide can be used as a raw material for lithium-nickel-cobalt-manganese composite oxide, but, according to the Examples, this coprecipitated nickel hydroxide has a tap density of 1.71 to 1.91 g/cm3, that is, less than 2.0 g/cm3, and hence it cannot be said that the coprecipitated nickel hydroxide has a sufficiently high density. On the other hand, Patent Literature 1 does not mention a specific numerical value of the specific surface area, does not clearly describe the optimization of the specific surface area, and does not mention the carbon content of the composite oxide at all. Therefore, even if this coprecipitated nickel hydroxide is used as a precursor, lithium-nickel-cobalt-manganese composite oxide having good battery characteristics cannot be obtained.
Furthermore, Patent Literature 2 discloses a method for manufacturing lithium-nickel-cobalt-manganese composite oxide, the method comprising: a step wherein, under the presence of a complexing agent in an aqueous solution of pH 9 to 13, a mixed solution of nickel salt, cobalt salt, and manganese salt, the solution substantially having an atomic ratio of nickel:cobalt:manganese of 1:1:1, is reacted with an alkaline solution under an inert gas atmosphere to be coprecipitated, whereby nickel-cobalt-manganese composite hydroxide and/or nickel-cobalt-manganese composite oxide, each substantially having an atomic ratio of nickel:cobalt:manganese of 1:1:1, are obtained; and another step wherein a mixture of hydroxide and/or oxide and a lithium compound is baked at a temperature of not less than 700 degrees C. so that an atomic ratio of the total of nickel, cobalt, and manganese to lithium is substantially 1:1.
Also in this Patent Literature 2, the obtained nickel-cobalt-manganese composite hydroxide has a tap density of 1.95 g/cm3, that is, less than 2.0 g/cm3, while has a specific surface area of 13.5 m2/g, which is very large. Furthermore, Patent Literature 2 does not provide any description about the carbon content of the composite oxide, and thus does not take an adverse effect on battery characteristics into consideration.
The capacity of a lithium-ion secondary battery is dependent on the mass of an active material filled in a battery, and therefore the achievement of lithium-nickel-cobalt-manganese composite oxide having a higher density and being more excellent in battery characteristics than a conventional one allows an excellent battery having a large electric capacitance with a limited volume to be obtained. Particularly, such lithium-nickel-cobalt-manganese composite oxide is advantageous in small secondary batteries for portable electronic equipment and batteries for automobiles, in both of which a space for a battery is limited.
As mentioned above, there has been desired nickel-cobalt-manganese composite hydroxide which allows lithium-nickel-cobalt-manganese composite oxide having excellent thermal stability to have a higher density and achieve improvement in battery characteristics.